Optical systems find widespread use in various industries. Optoelectronic systems in particular provide valuable monitoring and object detection capabilities, including in safety-critical applications. As an example, single- or multi-camera imaging systems may be used to monitor or “guard” the area around an industrial robot, metal press, or other hazardous machine, and to provide emergency-stop signaling in response to detecting the intrusion of a human being into the guarded, hazardous area.
Many machine vision systems use stereoscopic vision or multi-camera imaging, which involves two or more cameras having overlapping fields of view. By viewing the same object or objects from different viewing angles, the observed disparity between the positions of objects in respective ones of the multiple views provides a basis for computing distances to those objects. In an example case, a machine vision system uses stereoscopic image processing to detect objects in a monitored zone and to calculate the distances to those objects. Such detection may be safety critical, such as when the monitored zone represents a dangerous area or volume around an industrial machine and the machine vision system is relied upon for detecting the encroachment of objects into the monitored area. Here, the term “objects” can be broadly understood to include humans.
The accuracy and overall performance of a machine vision system depends on a number of critical mechanical alignments. Consider FIG. 1, for example, which depicts an example lens 6 and image sensor 8—“imager 8”—as might be included within a sensor head or other camera assembly of a machine vision system. Within the limits of the applicable manufacturing precision and tolerances, the camera assembly fixes the relative positioning of lens 6 and the imager 8, which may be Charge Coupled Device, CCD, or other pixel array.
These key alignments are better understood in the context of FIG. 2, which shows the image plane of the imager 8, in relation to the focal plane of the lens 6. Absent misalignment problems, the camera assembly fixes the image plane parallel to the focal plane and fixes the image plane at the proper distance d from the focal plane.
However, as a consequence of aging, thermal expansion, vibration, impact, etc., various kinds of mechanical misalignments may be introduced, such as a relative shifting or translation of the imager 8 relative to the lens 6 in a direction perpendicular to the optical axis. These side-to-side shifts, whether arising from movement of the lens 6 or imager 8, or both, move the optical center or origin R of the image plane off of the optical axis and out of alignment with the optical center or origin O of the focal plane.
Additionally, the lens 6 and/or imager 8 may shift along the optical axis. These shifts, which are referred to as longitudinal translations, change the distance d between the lens 6 and the imager 8. Longitudinal translations effectively change the magnification or focal length of the camera assembly.
As a further example of misalignment, the imager 8 may rotate about the optical axis. Such rotation effectively rotates the image projection from the lens 6 onto the image plane. Still further, any relative tilting of the lens 6 and/or imager 8 will cause the image plane to no longer be parallel with the focal plane. Tilting changes the image projection onto the image plane and, like the other, foregoing misalignments, degrade the ability of the machine vision system to accurately process the images obtained from its cameras.
Further issues arise in the context of focal quality and/or contrast. The lens 6 may have, for example, a focal quality profile wherein the focal quality of the lens 6 is highest at the lens center and lowest at the edges of the lens. FIG. 3 depicts an example focal quality profile for the lens 6.
While the focal quality profile of the lens 6 generally will be such that the minimum focal quality of the lens is acceptable, at least under nominal operating conditions, misting, fogging, oiling, dusting, etc., on the lens 6 and/or imager 8, will degrade the focal quality and/or contrast seen in the images acquired from the imager 8. Other environmental changes, such as changes in scene lighting may also degrade contrast or other image parameters.